1. Technical Field
Induction array tools and related techniques for formation resistivity logging are disclosed. More specifically, antennas for induction array tools for long transmitter-receiver off-sets, with increased sensitivities and related methods of use in resistivity measurements and formation or reservoir geometry are disclosed. Still more specifically, a deep triaxial resistivity tensor measurement tool is disclosed which assists in understanding geometries of hydrocarbon bearing formations. The disclosed tool can be used during the production phase of a reservoir to assist in understanding of induced or natural water fronts.
2. Description of the Related Art
Electromagnetic (EM) induction array toots are used in the oil and gas industry to determine the resistivity of earth formations surrounding a borehole. Because hydrocarbons have a higher resistivity than water or salt water, resistivity measurements are a primary means for locating or confirming the presence of hydrocarbon deposits in a formation. EM induction array tools work by using a transmitting coil or antenna (transmitter) to set up an alternating magnetic field in the earth formation. This alternating magnetic field induces eddy currents in the formation being evaluated.
A plurality of receiving coils or antennas (receivers), disposed at varying distances from the transmitter antenna is used to detect the current flowing in the formation. The transmitter-receiver spacings are typically less than a few meters. Multiple receiver antennas are used to focus formation current loops both radially (depth of investigation) and axially (vertical resolution). The magnitudes of the received signals arc essentially proportional to the formation conductivity and inversely proportional to the formation resistivity. Therefore, formation resistivities at varying distances from the borehole may be derived from the received signals. Varying depths of investigation are needed to evaluate the true formation resistivity and to compensate for borehole fluid invasion of the formation.
Conventional wireline and logging while drilling (LWD) EM induction array tools are equipped with coils or antennas that may function as both current sources or transmitters and receivers or sensors. The antennas of wireline EM induction tools are typically enclosed by an oil compensated housing (or tool body) constructed of a tough thermal-set or thermal plastic (insulating) material, e.g., a laminated fiberglass material impregnated with epoxy resin. The coils of LWD EM induction, array tools are generally mounted on metallic supports (collars) to withstand the harsh environments encountered during drilling.
The antennas of both wireline and LWD induction array tools are typically spaced apart from each other along the axis of the tool for different depths of investigation. In addition to borehole fluid invasion, formation anisotropy can also complicate resistivity logging and interpretation. Formation anisotropy results from the manner in which formation beds were deposited by nature. Formations containing hydrocarbons often exhibit anisotropy in formation resistivity. In such formations, the horizontal resistivity in a direction parallel to the bedding plane differs from the vertical resistivity in a direction perpendicular to the bedding plane. Further, formation analysis is also problematic for thin or highly laminated beds due to a lack of sensitivity of logging tools.
The antennas of induction array tools are typically of the solenoid-type that comprises one or more turns of an insulated conductor wire coil wound around a support. An antenna carrying a current can be represented as a magnetic moment proportional to the current and the area of the antenna. The direction and magnitude of the magnetic moment can be represented by a vector perpendicular to the plane of the coil.
In older induction and propagation logging instruments, the transmitter and receiver antennas were mounted with their magnetic moments aligned with the longitudinal axis of the instruments. As a result of this alignment, these instruments have longitudinal magnetic dipoles (LMD). When an LMD tool is placed in a borehole and energized to transmit EM energy, the induced eddy currents flow in loops around the antenna in the borehole and in the surrounding formation. The induced eddy currents flow in planes that are perpendicular to the tool axis and therefore the borehole axis. As a result, little or no eddy current flows up or down the borehole when the tool is centralized in the borehole.
More recently, the use of instruments incorporating antennas having tilted or transverse antennas, i.e., the magnetic dipoles of the antennas are tilted or perpendicular to the tool axis. As a result, these instruments have transverse or tilted magnetic dipoles (TMD). TMD instruments can induce eddy currents that flow up and down the borehole and, thus, provide measurements that are sensitive to dipping planes, formation fractures, or formation anisotropy. Modern induction tools typically include tri-axial arrays, in which the transmitter and receiver co-located groups or “banks” may each comprise three coils arranged in different orientations (typically in orthogonal x, y- and z-directions). For a given transmitter-receiver spacing one measures a complex conductivity matrix given as
      σ    apparent    =      (                                        σ            xx                                                σ            xy                                                σ            xz                                                            σ            yx                                                σ            yy                                                σ            zx                                                            σ            zx                                                σ            zy                                                σ            zz                                )  which can be inverted for horizontal resistivity (Rh), vertical resistivity (Rv), dip angle and azimuth assuming a dipping layered earth model.
While the TMD tools (including tri-axial tools) are capable of providing additional information about the formation resistivity, these tools are more strongly affected by the borehole for transmitter-receiver spacings less than a few meters, particularly in high contrast situations, when the mud in the borehole is more conductive than the formation. For transmitter-receiver spacings greater than a few meters, there is little borehole effect in the apparent conductivity tensor. Further, at least partially due to the lack of sensitivity of currently available wireline and LWD EM induction array tools, geologists and petrophysicists historically have found it necessary to visually analyze core samples extracted from zones of interest to assess complex or thinly laminated reservoirs and aid in the discovery of hydrocarbons. Because core samples can be difficult and/or costly to obtain, EM induction array tools with increased sensitivities are needed for the tensor conductivity volumetric measurements required to evaluate reservoir geometry and water/hydrocarbon boundary movement.